KR20000048502A - Static control with teos - Google Patents

Abstract

PURPOSE: A process for maintaining the electrostatic charges substantially at neutral levels thereby reducing or avoiding sheeting in a fluid bed gas phase olefin polymerization reactor. CONSTITUTION: A method for reducing sheeting in an operating gas phase fluidized bed olefin polymerization reactor, under olefin polymerization conditions, wherein the method comprises monitoring electrostatic level in the reactor, wherein the electrostatic level can be neutral, positive or negative; maintaining said conditions under which a negative electrostatic level develops in the reactor; feeding to the reactor a positive charge inducing quantity of a reagent selected from the group consisting of tetraethylorthosilicate, tetrapropyl orthosilicate and tetra butylorthosilicate and neutralizing said electrostatic level.

Description

Static control method using tetraethylorthosilicate {STATIC CONTROL WITH TEOS}

In a fluidized bed reactor, sheeting may occur upon shutdown of the reactor. Lamination is a phenomenon in which a layer of polymeric material is produced in the reactor wall (s). At least one of the theories that theoretically explains the deposition phenomenon is based on the generation of electrostatic charge (s) in the reactor as a result of polymerization reaction conditions. The occurrence of negative and positive electrostatic charges may be related to the lamination phenomenon. In addition, the occurrence of negative and positive static charges is affected by the temperature variations present along the reactor walls. The sheets are relatively thick and range from about 1/4 inch to 1/2 inch. This sheet may range from several square inches to several square feet. Lamination phenomenon was observed to increase with increasing static electricity level. The reason for this accelerated lamination may be due to the lack of cooling means at the wall location resulting in hot spots. Such hot topical sites tend to melt and fuse the resin into larger layers.

FIELD OF THE INVENTION The present invention relates to a method of reducing lamination, in particular negative static, during a polymerization reaction (or copolymerization reaction) in a fluidized bed gas phase reactor. Negative static electricity is well known to those skilled in the art and can be measured in the form of voltage or current. Under the fluidized bed gas phase polymerization reaction conditions, lamination may occur. If positive static occurs in the gas phase reactor, the positive static electricity is usually neutralized using a method of adding water. The present invention relates in particular to a method for neutralizing negative static electricity. The process of the present invention includes feeding tetraethylorthosilicate at 16 ppm to 40 ppm (based on the ethylene feedstock stream) as an auxiliary feedstock for the fluidized bed reactor to reduce negative static electricity. Tetraethylorthosilicate can actually act to induce positive static electricity, thereby neutralizing negative static electricity. Tetraethylorthosilicate may also react with species in which the reactor causes positive static electricity. The addition of tetraethylorthosilicate and water can act as a complementary pair of reagents to neutralize reactor static and thereby reduce the phenomenon of deposition in the gas phase reactor.

Thus, a method for reducing the stacking phenomenon in a gas phase reactor provided in accordance with the present invention comprises the steps of: measuring the electrostatic degree in the reactor, adding tetraethylorthosilicate if the electrostatic degree is negative, positive electrostatic degree If is shown, for example, water is added, and subsequently monitoring the occurrence of static charge in the reaction chamber to maintain a neutral static charge in the reactor.

The present invention relates to a low pressure polyethylene production process performed under a pressure of 1000 psi or less. Processes for producing polyethylene under low pressure employ fluidized bed gas phase polymerization reactors operating at pressures of up to 1000 psi, preferably up to 400 psi, typically from 100 psi to 350 psi. Low density, in particular linear low density polyethylene [LLDPE, density 0.94 or less] and high density polyethylene [density 0.94 or more] can both be produced in a fluidized bed gas phase reactor.

1 shows a fluidized bed gas phase reactor that can be used in the process of the present invention.

Conditions in Fluidized Bed Reactor

Ethylene polymers and copolymers of ethylene with one or more C ₃ -C ₁₀ alpha-olefins such as propylene, butene-1, pentene-1, hexene-1 can be prepared according to the invention. Therefore, not only the copolymer containing two monomer units but a ternary copolymer containing three monomer units is also possible. Specific examples of such a copolymer include ethylene / 1-butene copolymer, ethylene / 1-hexene copolymer and ethylene / 4-methyl-1-pentene copolymer. Ethylene / 1-butene and ethylene / 1-hexene copolymers are the most preferred copolymers that can be polymerized using the process and catalyst of the present invention. The ethylene copolymers prepared according to the invention preferably contain at least about 80% by weight of ethylene units.

In the polymerization reaction of this invention, hydrogen can be used as a chain transfer agent. The hydrogen / ethylene ratio used ranges from 0 mole to 2.0 moles of hydrogen per mole of ethylene in the gas phase. Any gas that is inert to the catalyst and reactants may be present in the gas stream.

A fluidized bed reaction system that can be used to practice the method of the present invention is schematically illustrated in FIG. 1. Referring to FIG. 1, the reactor 10 is composed of a reaction zone 12, a deceleration zone 14, and a distribution plate 20. Adhesion may occur in all low temperature regions (areas in the reactor below which the temperature at which any component (s) in the gas phase reactor is present as a liquid rather than a gas), but contamination of the distribution plate is most easily detected, This is because the pressure drop across the distribution plate increases sharply due to flow restriction. In addition, this flow restriction results in a change in fluidization pattern that contaminates the reactor walls. The lowest temperature in the reactor loop is at the reactor inlet below the distribution plate. Other regions representing the coldest regions in the fluidized bed reactor system include chillers and piping between the chillers and the bottom.

Reaction zone 12 comprises a small amount of catalyst particles fluidized by a continuous flow of resulting polymer particles and polymerizable reforming gaseous components. In order to maintain the effective fluidized bed, the mass gas flow rate through the fluidized bed, and is greater than the minimum flow rate required for fluidization, G _mf 1.5~10 times preferably of, and is 3-6 times more preferably of G _mf. G _mf is an abbreviation of the form commonly used for the minimum gas mass flow rate required to achieve fluidization, see CY Wen and YH Yu, "Mechanics of Fluidization", Chemical Engineering Progress Symposium Series, Vol. 62, p. . 100-111 (1966). The distribution plate 20 is used for the purpose of passing recycle gas through the bed at a rate sufficient to maintain fluidization at the fluidized bed base. Fluidization is achieved by a high gas recycle rate to and through the bed, for example by a gas recycle rate that is about 50 times the make-up gas feed rate. Supplemental gas is fed to the bed at a rate equivalent to the rate at which the particulate polymer product is formed by the reaction. The composition of the make-up gas is measured by the gas analyzer 16, the feed point of which is located above the bed. The composition of the make-up gas is continuously adjusted to keep the gas phase composition substantially rectified in the reaction zone.

A portion of the gas stream that does not react in the bed (recycle gas) passes through the deceleration zone 14 where the entrained particles pass through the cyclone 22 (optional component) and the filter 24 (optional component). Through the heat exchanger (26) and back to the bed after passing through the heat exchanger (26). The distribution plate 20 is used for the purpose of diffusing the recycle gas through the bed at a rate sufficient to maintain fluidization. The distribution plate may be a screen, a slotted plate, a perforated plate, a bubble cap plate, or the like.

Fluidized bed reactors operate at temperatures below the sintering temperature of the polymer particles. When producing an ethylene copolymer in the process of the present invention, the operating temperature is preferably 30 ° C to 115 ° C, most preferably 75 ° C to 95 ° C. In order to produce a product with a density of 0.91 to 0.92, a temperature of 75 ° C to 90 ° C is used, and a temperature of 80 ° C to 100 ° C is used to prepare a product having a density of 0.92 to 0.94, and a density is 0.94 to A temperature of 90 ° C. to 115 ° C. is used to produce a product that is 0.96.

Fluidized bed reactors operate at pressures up to about 1000 psia, preferably at pressures from about 150 to 250 psia, and are good in terms of heat transfer when operated at pressures higher than this range. This is because the unit volume of the heat capacity increases.

Partially or fully activated catalyst is injected into the bed at a point on the distribution plate at a rate equivalent to its consumption rate. Since the catalyst used in the fluidized bed is highly active, the injection of a fully activated catalyst into the region below the distribution plate may cause a polymerization reaction at that position, which may cause the distribution plate to be blocked in some cases. Instead, injection into the bed helps to distribute the catalyst throughout the bed and prevents the formation of localized regions with high catalyst concentrations.

The rate of polymer production in the bed is controlled according to the catalyst injection rate. Since the rate of generation of reaction heat changes when the rate of catalyst injection changes, the rate of exotherm can be changed by adjusting the temperature of the recycle gas. Of course, it is necessary to fully instrument both the fluidized bed and the recycle gas cooling system in order to detect temperature changes in the bed so that the operator can adjust the temperature of the recycle gas appropriately.

Since the exotherm rate is directly related to the formation of the product, the measurement of the temperature rise of the gas crossing the reactor (difference between the injection gas temperature and the exhaust gas temperature) is a determinant of the rate of particulate polymer formation under constant gas velocity. Under a given set of operating conditions, the fluidized bed is maintained at a substantially constant height by recovering a portion of the bed at a rate equivalent to the rate of formation of the particulate polymer product.

The catalyst used for the catalytic polymerization in a fluidized bed reactor is an important factor in the formation of electrostatic charge and lamination, since the catalyst can cause the formation of a charge. The catalyst is usually a titanium and / or vanadium containing catalyst and a chromium catalyst. Such catalysts are usually supported on silica, silica / alumina or magnesium oxide supports. Catalysts reported to cause negative static in a fluidized bed gas phase reactor are disclosed in Example 3 of US Pat. Nos. 4,803,251 and 2,5, which are incorporated herein by reference. Such negative static electricity may cause a lamination phenomenon as described above. Using the catalyst disclosed in US Pat. No. 4,803,251, contaminants such as oxygen have been found to increase positive electrostatic charges, while water causes negative charges. Negative static can be measured in a reactor by monitoring the voltage on a metal spike insulated with ceramic and connected to an ammeter by a conductor. This configuration is similar to the Auburn Triboflow meter on the market. In comparison, oxidized hexene (carbonyl / peroxide) and water have not been shown to have an effective effect on reactor static electricity when using the catalyst systems disclosed in US Pat. Nos. 5,336,652 and 5,470,812. In the gas phase fluidized bed polymerization reaction using such a catalyst, the lamination phenomenon is not easy to occur. The above US patents are also incorporated herein by reference. Catalysts containing 0.69 mmol / g of tetraethylorthosilicate (TEOS), prepared according to U.S. Pat. It was proved to represent. However, if the TEOS concentration is lowered to 0.44 mmol per gram of silica, the reactor static will average 2-4 nA. In light of this trend, it was determined by testing whether TEOS neutralizes positive static in gas phase reactors for other catalyst systems as well.

Static in the reactor is measured using metal spikes that are insulated with ceramic material and connected to the ammeter by conductors. This is similar to the Triboflow meter sold by Auburn.

Values of the order of -2 nA to -4 nA (negative static electricity) are sufficient values necessary to use the method of the present invention. Experimental results of the present inventors have confirmed that TEOS (tetraethylorthosilicate) can remove negative static electricity in the gas phase reactor.

Usually, positive static electricity is controlled by the method of adding water. Thus, the addition of TEOS and water is a complementary pair to neutralize the static in the gas phase reactor to reduce lamination.

The amount of TEOS added is in the range from 16 to 40 ppm, preferably from 16 ppm to 30 ppm, more preferably from 20 ppm to 30 ppm, most preferably from 22 ppm to 25 ppm. TEOS can be added by injecting TEOS as an auxiliary feedstock similar to the case of controlling the alkylation reaction. This auxiliary feedstock is injected as a liquid in the recycle stream, in which TEOS evaporates and moves in gaseous form in the reactor. The feed rate of 16-40 ppm by weight is based on the ethylene stream fed to the reactor, multiplied by 1,000,000 by the weight of TEOS divided by the weight of ethylene.

When the reactor is monitored to measure electrostatic formation in the reactor, an effective amount of reagent is supplied to the reactor to neutralize the electrostatic formation. Usually, to control positive static electricity, water is added at a concentration of 10 ppm by weight. However, other reagents such as ketones having 1 to 8 carbon atoms may be used. This additive is used in the ethylene feedstock to the reactor. While not wishing to be bound by any particular theory, the inventors of the present invention have set forth the following theory to explain the lamination mechanism. Static electricity is generated by frictional contact between two different materials, namely polyethylene resin and carbon steel. Each charged resin particle sets its respective electric field in the fluidized bed. The strength of this electric field increases from zero at the axial position to the maximum strength at the wall position.

This longitudinally oriented electrostatic force always competes with fluidizing power which tends to randomly distribute the layers. If the layer generates a sufficiently high electric field, the movement of particles in the wall is disturbed. This partial fluidization loss causes flat polymer material to form on the walls due to insufficient cooling.

TEOS was diluted with isopentane and fed to the bottom of the reactor at a position below the distribution plate. The TEOS concentration was assessed using 0.25 ppm to 40 ppm. Each time TEOS was fed into the reactor, static electricity tended to rise to a positive value, which was reproduced several times. As the TEOS supply increased further, the positive electrostatic degree gradually increased. The table below summarizes the positive electrostatic charge caused by various TEOS concentrations.

TEOS supply (ppm) Duration (minutes) Static charge (nA) 0.24 0.24 40 0.5 0.48 40 0.5 2.40 40 0.7 16.00 40 0.9 40.00 60 1.5

Without TEOS feedstock, the electrostatic reference was 0.0-0.5 nA. As the TEOS feed increased, the positive static charge increased to 0.9 nA at 16 ppm and then to 1.5 nA at 40 ppm. Immediately after stopping the TEOS supply, the static electricity decreased over a period of 30 minutes and, in some cases, stabilized near the reference value (0.0-0.5 nA). All tests were performed using a titanium 40/20 catalyst disclosed in US Pat. No. 5,139,986, incorporated herein by reference, and co-feeding TMA (trimethylaluminum). The 40/20 formulation is formulated with a DEAC: THF molar ratio of 0.40 and a TEAL: THF molar ratio of 0.20 in the formulation. At concentrations ranging from 0.25 ppm to 40.0 ppm, TEOS had no effect on catalyst activity, method response or MFR.

Accordingly, the present invention provides a method that fully satisfies the above objects and advantages of the present invention. While the invention has been described in connection with specific embodiments, those skilled in the art will clearly appreciate many modifications, variations, and variations of the invention from the foregoing detailed description. Accordingly, all such modifications, changes and variations are included in the protection scope of the invention as defined by the spirit of the invention and the appended claims.

Claims (9)

A method of suppressing sheeting in a gaseous fluidized bed olefin polymerization reactor in operation, Monitoring the degree of static electricity in the reactor whether it is neutral, positive or negative, If the electrostatic degree in the reactor is negative, adding tetraethylorthosilicate in an amount causing a positive charge and monitoring the positive electrostatic degree in the reactor, Adding a reagent in an amount that causes a negative charge if the static in the reactor is positive and Maintaining a neutral static charge in the reactor Characterized in that it comprises a. The method of claim 1 wherein the amount of tetraethylorthosilicate is in the range of 16 ppm to 40 ppm. The method of claim 1, wherein the amount of reagent causing the negative charge is in the range of −2 nA to −4 nA. The method according to claim 1, wherein tetraethylorthosilicate is supported on silica. The process of claim 1, wherein TEOS is added to another Ziegler catalyst. The method of claim 1 wherein the silicate source is tetramethylsilicate, tetrapropylsilicate or tetrabutylsilicate. The method of claim 1 wherein the silicate source is another liquid hydrocarbon. The method of claim 1, wherein the silicate liquid is fed onto a distribution plate. The method of claim 1, wherein the silicates are fixed chemically or physically to the reactor walls.